A wind turbine converts wind power into electrical energy through a generator driven by the blades of the wind turbine under windy conditions. In some wind turbines, the generator comprises a stator and a rotor both having multiphase windings, such as a double-fed induction generator, a cascade-regulated generator, an induction or a synchronic generator, or even a dc generator.
When having a rotor rotating in relation to a stator of a generator, electricity has to be transferred to or from the rotor. Electricity may be transferred by means of a slip ring assembly also called a rotary electrical interface, rotating electrical connector, collector, or swivel or electrical rotary joint, enabling the rotor to receive or return electricity while rotating in relation to the stator. When having a rotor with a multiphase winding, a control system within the wind turbine is able to regulate the frequency, so that the wind turbine produces at a predetermined frequency, e.g. approximately 50 Hz.
Typically, the slip ring assembly comprises a rotational slip ring unit rotating with the rotor and having conductive rings or bands. Furthermore, the slip ring assembly is in contact with brushes having sliding contact with the rings for transferring electricity from the rotor. Thus, the electricity is transferred from the slip ring unit to the rotor windings in order to control the electrical field in the generator. The electricity is transferred to the slip ring unit by means of brushes bearing against the conductive rings, typically three conductive rings—one for each electrical phase.
Upon mounting of the brushes in the brush holders, it is important that there is full contact between brush(es) and the conductive ring(s) in order to avoid point contact leading to too high current density on the brushes. The contact surface of each brush is shaped as an arcuate surface, typically around 1 cm×5 cm, where the radius of curvature is aligned with the radius of curvature of the slip ring. Indeed, due to the limited visual access to the bearing surfaces and as should be clear by looking at e.g. FIGS. 2 and 3, it may be difficult to ascertain visually whether there is full contact between slip ring and brush, and significant guess work may be involved. The consequence is that a full initial contact between the brush and the conductive ring may not be reliably achieved.
It is an object of the present invention to facilitate a correct mounting of the brushes to establish full contact with the slip ring.